The present invention relates to a method for rolling a workpiece with a rolling tool with several rolling elements arranged in hydrostatic bearings. The present invention further relates to a rolling tool with several rolling elements arranged in hydrostatic bearings, and an arrangement of such a rolling tool and a workpiece.
To ensure clarity, it is necessary to establish the definition of several important terms and expressions that will be used throughout this disclosure. The term xe2x80x9crolling tool with several rolling elements arranged in hydrostatic bearingsxe2x80x9d refers to any tool which is suitable for exerting pressure forces onto a workpiece by way of rolling elements. This includes, in particular, planishing and solid-rolling. These are technologically very effective methods for smoothing surfaces, in particular of metallic components, and for solidification of the skin in order to increase fatigue resistance.
For planishing and solid-rolling, i.a., hydrostatic rolling tools are used, as described in European Pat. No. EP 0 353 376 A1. Highly loaded workpiece contours arranged on the inside are usually not accessible to these tools. Other methods must be used for contours which are so small and complex that known tools, due to their design size, do not fit into the regions to be worked. Furthermore, the tools, which usually only comprise one ball or roller or several balls or rollers, require a three-dimensional movement sequence which completely matches the workpiece contour, said movement sequence having to be carried out by the machine control system. This requires an expensive control system which most of the time turns out to be impracticable.
The shot-peening process is thus often used for skin solidification of complex contours, as there is a lack of better methods. The shot-peening process is associated with the disadvantage of shallow penetration depth and thus reduced effectiveness compared to roll methods. Moreover, the confined spaces in the case of complex contours often result in the used peening material banking up instead of flowing away freely. This further reduces the effectiveness of the shot-peening process.
Complex contours can also be rolled with single-roll mechanical tools or hydrostatic tools. The rolling process then takes place at an action point between the roll and the tool surface. The area of the workpiece can be achieved by two-dimensional movement of the tool or the workpiece. This includes, for example, rotation of the tool and advance of the tool. In the case of complex areas such as for example free-form areas, a feed movement in the third axis is superimposed.
Occasionally, rolling tools with several hydrostatic roll elements arranged on a circumferential line of a tool are used. These tools are suitable for treating interior circular lines, and, if the tool or the workpiece carries out a feed movement, they are also suitable for treating interior cylinder surfaces. However, the use of these tools is limited to applications of this kind.
It would therefore be desirable and advantageous to provide an improved method for rolling a workpiece with a rolling tool with several rolling elements arranged in hydrostatic bearings, which obviates prior art shortcomings and allows simple and effective treatment of workpieces.
According to one aspect of the present invention, in a method for rolling a workpiece, a rolling tool has several rolling elements which are arranged in hydrostatic bearings, wherein during rolling, only some of the rolling elements engage the workpiece, while those rolling elements that are not in engagement with the workpiece essentially seal off the hydrostatic bearing.
Unlike conventional methods that use rolling tools with several rolling elements arranged in hydrostatic bearings, whereby all rolling elements are in constant engagement during the entire rolling process, the rolling method according to the present invention is based on the recognition that there are new possibilities for producing hydrostatic rolling tools if sealing is provided for the hydrostatic bearings which are not engaged. This sealing action is achieved in a simple way by those rolling elements which do not engage the workpiece in order to assume the sealing function of the associated hydrostatic bearings. As a result, the control operation is considerably facilitated because the lateral movement component of the rolling element can be exploited for sealing the hydrostatic bearing during interaction with the workpiece and during the disengagement of the rolling element.
It has been shown that the hydrostatic pressure acting on the rolling elements is normally sufficient to press the rolling element against a circumferential area which acts as a valve seat.
According to another feature of the present invention, the rolling tool may be moved in a linear or rotational manner. Thus, any interior or exterior surfaces of the workpiece can be manipulated by the rolling tool. The arrangement of the hydrostatic bearings on a tool body can be suited to the profile of the workpiece to be worked on. However, it is also possible to design rolling tools for universal use which tools can be guided so as to move in a translational and/or rotary way along the workpiece surfaces.
An advantageous variant of the process according to the invention provides for a repeated treatment of a same position of the workpiece, when the rolling tool is moved linearly in one direction. It is also possible to roll closely adjacent areas of the workpiece, using rolling elements which are successively guided over the workpiece. When using a plurality of rolling elements, it is also possible to repeatedly roll the same positions of the workpiece, using different rolling elements of the same tool. In this way, the rolling quality can be enhanced through suitable selection of the tool size or number of the rolling elements.
According to another aspect of the present invention, a rolling tool includes several rolling elements arranged in hydrostatic bearings in which the rolling elements are arranged so as to be distributed over an area.
Unlike conventional rolling tools having rolling elements arranged along a straight circumferential line, the rolling tool according to the invention provides for the rolling elements to be arranged so as to be distributed over an area of the rolling tool. In this way, the rolling elements are able to roll in a single rolling operation, not only a line, but also an area of the workpiece.
According to another feature of the present invention, the area may be curved at least in one direction. The area can thus be suited to the contour of the workpiece or allows between the tool and the workpiece an optimal engagement that is easy to control.
The area on which the rolling elements are arranged in spaced-apart relationship may also include concave and convex kinks, wherein rolling elements are suitably arranged in the region of the kinks. In this way, the rolling tool can be suited to specific workpieces for optimal rolling operation.
The rolling elements can be arranged on the rolling tool along parallel lines or along parallel circumferential lines. Currently preferred is a configuration in which those rolling elements disposed successively in rolling direction are arranged in offset relationship. As a consequence of the offset arrangement of the rolling elements, no single position of the workpiece is rolled repeatedly, but instead, closely adjacent positions of the workpiece are rolled so as to roll an area as evenly as possible.
When the number of rolling elements arranged in hydrostatic bearings on a rolling tool exceeds five bearings which are supplied together with hydraulic fluid, then the quantity of fluid required during the rolling process increases to such an extent that effective rolling operation is no longer possible. For this reason, according to the invention, it is proposed that, in the case of rolling tools having several rolling elements arranged in hydrostatic bearings, each of the hydrostatic bearings includes a circumferential seal in the region of the rolling elements.
In conventional hydrostatic bearings, it was possible to stop the supply of hydraulic fluid for all bearings together. However, individual control of the supply of hydraulic fluid to individual bearings was not possible. In accordance with the present invention, a circumferential seal may be provided in the region of the rolling elements. The arrangement of a seal in this position allows opening or closing of the seal through lateral movement of the rolling elements. Thus a hydrostatic bearing can be opened or sealed by interaction between rolling element and workpiece, without the need for an additional control system or regulating system.
According to another feature of the present invention, the hydrostatic bearings may include a sealing gap which is so dimensioned as to act as hydraulic throttle. It is the basic function of the sealing gap to enable or stop the through-flow of hydraulic fluid. The design as a hydraulic throttle has the advantage that even when the sealing gap is open, only a quantity of hydraulic fluid, defined by throttle action, flows through the sealing gap and reaches the region between the tool or the rolling element and the workpiece. In this way too, the quantity of hydraulic fluid issuing through the multitude of hydrostatic bearings is reduced.
According to still another aspect of the present invention, an arrangement includes a rolling tool with several rolling elements arranged in hydrostatic bearings, and a workpiece, whereby some of the rolling elements protrude beyond the hydrostatic bearing commensurate with a distance between the hydrostatic bearing and the workpiece, while some other rolling elements protrude beyond the hydrostatic bearing by more than the distance.
In this arrangement, the position of the rolling element thus changes in dependence on whether or not the rolling element engages the workpiece. Thus, the position of the rolling element can be exploited for sealing and controlling a throttle. When there is particular firm bearing pressure between the rolling element and the workpiece, the throttle between the hydrostatic bearing and the rolling element is opened up wide, with a considerable quantity of hydraulic fluid issuing as a consequence. However, if the bearing pressure between the rolling element and the workpiece is low, the throttle is opened only slightly so that only a relatively small quantity of hydraulic fluid leaves the hydrostatic bearing. Finally, when the rolling element no longer engages the workpiece, the throttle turns into a seal, preferably completely sealing off the issue of hydraulic fluid.